What are the symptoms of osteopenia for 18 years old men?

18 year-old men do not have osteopenia, unless there is severe underlying disorder or you are on high dose steroids. You may wish to consult a doctor for your symptoms. In the meantime, start exercising. Begin with low intensity exercise such as walking and gradually increase the intensity so that you sweat profusely. It may take a few months for the full effect. Wish you good health!

Unfortunately, there are usually no symptoms until the disease is far progressed and you start developing weird fractures. You are so young, shouldn't be much of a concern unless you've been doing something like using steroids. If something like that is the case, get a simple bone density exam done. Painless, will give you excellent read on the status of your bones.